Place the egg and cilantro in a food processor or blender. Start processing and slowly stream in 1/2 cup of the oil. Add the garlic, 1/2 teaspoon of the salt, and 4 turns of the pepper, and stream in the remaining 1/4 cup of the oil. Process until thoroughly emulsified into a mayonnaise.
Pour the mayonnaise into a large bowl and add the potatoes, onions, and the remaining 1/2 teaspoon salt and 3 turns pepper. Toss to combine thoroughly, cover, and refrigerate for up to 24 hours.
Yield: about 5 1/2 cup, 5 generous salad servings